If your locale is not available, you may install a language pack from an earlier version. They are compatible, however you may find many untranslated strings when trying to use it. How to create a new translation?
Get the ZIP file for the language you want and unzip it into the /blogs/locales folder.
If possible unzip your lang-pack into the right folder before install.
Make a ZIP file named xx-YY and containing exactly the contents of a subfolder of the /locales folder.
Please send the file to François with all relevant details, especially: